1. Field of the Invention
The present invention relates to novel mixed salts of polyalkyl vinyl ether maleic anhydride and their use in dental adhesives to secure dental prostheses, and a process for their preparation.
2. Technology Review
Commercially available dental adhesives are sold as powders, creams or liquids. The essential function of dental adhesives is to give the prosthesis the most secure seat and the best possible adhesion. Saliva plays an important role in the adhesion process because in the presence of saliva the adhesive must establish a connection between the prosthesis and the palate by generating cohesive and adhesive forces. Therefore, adhesives must contain substances which swell upon contact with saliva and form an elastic film on the base of the proshesis. Substances suitable as adhesives must not irritate mucous membranes, and must be toxicologically acceptable, particularly with respect to heavy metals since these may enter the gastro-intestinal tract together with the saliva. Further, substances suitable as adhesives must not contain any harmful microbiological impurities or contaminants and must not chemically react with the prosthesis material. Finally, suitable adhesives should be free of annoying odors and taste, and provide the longest possible duration of adhesion while being safely removable from the palate and from the prosthesis without leaving residues.
In the past, natural swellable substances were used as adhesives. These were natural polymers, e.g. polysaccharides, which with water form highly viscous solutions or suspensions that have elastic properties. In modern formulations, synthetic polymers are combined with these natural substances or are employed alone.
For example, published application DE-OS 1,909,209 discloses a natural polymer, tragacanth, or karaya gum in combination with sodium methyl cellulose as the adhesive component. The natural polymer, tragacanth, swells in the aqueous environment of the mouth and is able to promote adhesion only in this state. A drawback is that the magnitude of the adhesive force is dependent on the quantity of saliva in the each individual's mouth so that the adhesive force is different from user to user.
Published application DE-OS 2,025,268, which appears to correspond to U.S. Pat. No. 3,575,915, discloses a dental adhesive cream which contains polyvinyl acetate and a softener as its adhesive component. The softener makes it difficult to remove the adhesive cream from the prosthesis for cleaning purposes and has an irritating effect on the mucous membranes of the oral cavity. Moreover, if the cream contains too much softener, it becomes too soft and its adhesion is insufficient even for normal stresses.
Published application DE-OS 1,467,795 discloses an adhesive cream in which polyvinyl pyrrolidone is employed as the adhesive component. A grave drawback of this adhesive cream is that the additives employed to neutralize its taste, particularly citrate derivatives, adversely influence the taste sensation.
The production and use of mixed Ca/Na salts of the maleic acid anhydride/methyl vinyl ether copolymer is disclosed in U.S. Pat. No. 3,003,988. This powdered material is very well suited as an adhesive component in adhesives; however, it has the decisive drawback that noticeable fluctuations in adhesion may occur due to the process used to dry the salt.
In published application DE-OS 2,133,709, the drawbacks mentioned above of U.S. Pat. No. 3,003,988 are purportedly removed. This published application teaches that the powdered copolymer salt produces its best adhesion only if it is dried in a fluidized bed furnace. Its maximum adhesion is attained if the moisture content of the powdered salt lies at 10%.+-.1% and is kept constant.
A process for manufacturing the basic polymer maleic anhydride (MA)/methyl vinyl ether (MVE) employed for the salt formation in published application DE-OS 2,133,709 and U.S. Pat. No. 3,003,988 may be found in U.S. Pat. No. 2,782,182 and U.S. Pat. No. 2,047,398, incorporated herein by reference. The radical polymerization of the monomers, maleic anhydride and methyl vinyl ether occurs in a solvent, for example benzene or methylene chloride, and is possible only when benzene is used to obtain an end product which has a sufficiently high degree of polymerization. After drying, the copolymer produced contains large quantities of benzene which cannot be removed by distillation or simple drying. Therefore, these polymers cannot be employed for the production of the respective Ca/Na salts. In some countries, it is forbidden by law to use such materials for medical/cosmetic applications such as dental adhesives.
An improved method of producing the respective copolymers free of benzene was developed by the assignee of the present invention and is disclosed in published application DE-OS 3,712,265. Such material is particularly suitable for the production of the novel Al/Ca/Na salts according to the present invention and also for the Ca/Na salts disclosed in U.S. Pat. No. 3,003,988 and in published application DE-OS 2,133,709.
European Patent Publication No. EP A2 0,265,916, which appears to correspond to U.S. Pat. No. 4,758,630, reports dental adhesives which contain zinc and/or strontium salts of the MA/MVE copolymer in addition to the Ca/Na salt of this copolymer. The zinc salt is preferred in the adhesives disclosed. If the adhesive contains either the zinc or the strontium salt, and also the calcium/sodium salt of the MSA/MVE copolymer, the adhesive effect is purported to be substantially better than in formulations containing only the Ca/Na salt. A serious drawback of this adhesive is that it contains the toxic metals zinc and strontium. Additionally, the salt produced according to Example 1 of European Patent No. EP A2 0,256,916 must be dried for 16 to 18 hours at 70.degree. C. The long drying time and the use of highly diluted aqueous suspension in the manufacturing process for these zinc and strontium salts are uneconomical.
It is the object of the invention to provide adhesive components for dental adhesives that exhibit substantially improved adhesion, do not contain any physiologically objectionable heavy metals, and can be manufactured economically.